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Re: [PATCH 2/4] x86/P2M: relax guarding of MMIO entries

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 1 Sep 2021 10:08:44 +0200
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  • Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 01 Sep 2021 08:09:23 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 31.08.2021 17:38, Jan Beulich wrote:
> On 31.08.2021 17:25, Andrew Cooper wrote:
>> On 31/08/2021 14:26, Jan Beulich wrote:
>>> On 31.08.2021 15:16, Andrew Cooper wrote:
>>>> On 30/08/2021 14:02, Jan Beulich wrote:
>>>>> Further permit "access" to differ in the "executable" attribute. While
>>>>> ideally only ROM regions would get mapped with X set, getting there is
>>>>> quite a bit of work. Therefore, as a temporary measure, permit X to
>>>>> vary. For Dom0 the more permissive of the types will be used, while for
>>>>> DomU it'll be the more restrictive one.
>>>> Split behaviour between dom0 and domU based on types alone cannot
>>>> possibly be correct.
>>> True, but what do you do.
>>>> DomU's need to execute ROMs too, and this looks like will malfunction if
>>>> a ROM ends up in the region that HVMLoader relocated RAM from.
>>>> As this is a temporary bodge emergency bugfix, don't try to be clever -
>>>> just take the latest access.
>>> And how do we know that that's what is going to work?
>> Because it's the pre-existing behaviour.
> Valid point. But for the DomU case there simply has not been any
> pre-existing behavior. Hence my desire to be restrictive initially
> there.

Further to this: Using the last-value-set approach also puts us at
risk of running into a similar issue again when the ordering of
some operations changes elsewhere.




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